The present invention relates to fasteners in general and, more particularly, to fasteners that employ expandable elements to engage the walls of the structure in which the fastener is used and a positive lock that keeps the fastener from pulling through the structure.
U.S. Pat. No. 3,192,820 to Pitzer describes a quick release pin that employs a plurality of alternate male and female, split ring bushings which cooperate to engage the bores of workpieces to hold the workpieces together. In greater particular, the pin has a head and means such as a cam to apply a tensile force on a core shaft on which the bushings mount and a balancing compressive force on the bushings. The male bushings have conical surfaces facing outward away from the axis of the core shaft. The female bushings have conical surfaces facing inward, towards the axis of the core shaft. Male and female bushings are placed alternately so that the male bushings upon the application of the compression force wedge against the female bushings and expand them tightly against the wall of the structure being fastened. Simultaneously, the female bushings press the male bushings into tight engagement with the core shaft. There are variants of this quick release pin, but all of them use the split ring bushings to effect a radial compressive lock between the fastener and the structure being fastened.
Examples of the variants include blind bolts that employ a nut on a threaded end of the core shaft to effect the compression of the bushings and the expansion of the female bushings against the walls of structure. Means such as a wrenching socket in the core shaft can be used to constrain rotation of that shaft while torque is applied to the nut. Headed bolts may be used in place of the blind bolt where blind applications are not necessary. In this configuration, a standard headed bolt serves for wrenching on one side of the structure and a nut threaded on the other end of the bolt functions to compress the bushings and expand the female bushings into their locking relationship with the structure. Clamp-up bolts are another variant. They employ a core shaft and two threaded ends to permit, first, an expansion of the female bushings into engagement with the structure and, second, a clamp-up of the workpieces of the structure.
U.S. Pat. No. 3,192,820 also describes a positive locking ring for a fastener that expands over a male element in response to the application of an axial compressive force. The ring is located to expand for bearing on the structure outside the hole that receives the fastener to provide a positive interference lock between the fastener and the structure that prevents the fastener from being pulled from the structure towards the side opposite the ring.
The advantages of the split ring bushing fastener are many. They are simple to use and have high shear strength, the equivalent of solid shank bolts of the same diameter. They effect positive alignment between holes in two workpieces. They can provide clamp-up of attached parts. They are easy to install and remove, especially with a cam handle. They are very resistant to vibration. They adapt to blind operation and effect reasonable tensile strength. As a result, they remove or reduce fretting or wear due to vibration or movement. They provide an interference fit with the resultant desirable wall preload and its attendant improved fatigue resistance.
These expandable fasteners find many applications. For example, they may be used on auxiliary fuel tanks for aircraft where, through the mere flipping of a lever, the tanks can be released. They find application in helicopter blades and in mounting military ordnance. In all of these applications it is very important that the fastener secure its load so that it does not fall off, and in all of these applications, load can be subject to severe vibration induced accelerations.
It is desirable, however, to provide the expandable bushing fastener with a positive lock to assure that the fastener does not draw out of the structure fastened. Such a positive lock is effected by an adequate interference between the fastener and the structure in the direction against which axial pullout resistance is desired, typically on the side opposite the active element used to expand the female bushings. Such a positive lock should be free from the effects of vibrations, something to which threaded parts are quite often subject. The lock should be positive in the sense that it is not lost by radial forces on the lock. Thus, springs in the force train should not be used.